1. Field of the Invention
The present invention relates to, in an electronic imaging device such as a digital still camera, an imaging apparatus, an image processing apparatus, and an image processing method which can generate a bulb photography image by image composition and which perform still photographing of a celestial body by moving an image sensor or a camera itself in the same direction as a moving direction of the celestial body by a diurnal motion.
2. Description of Related Art
There have been many photographers who take photos of the celestial body since the age of film cameras before the digital cameras were put on the market. Celestial photographing is frequently performed under long exposure, since subjects are extremely dark except the sun and the moon. If photographing is performed under long exposure with the camera fixed to a tripod or the like, the celestial body moves with the diurnal motion, and an image in which a trajectory of the celestial body flows linearly is obtained. Moreover, for bright ones in the heavenly bodies such as the sun and the moon, photographing can be performed with appropriate exposure without such long exposure for the other dark celestial bodies. However, in the case of photographing using a telephoto lens, since an image surface moving speed of the subject image on the imaging surface (film surface) is fast, the image flows.
Thus, a method is performed in which the camera is mounted on an equatorial telescope moved so as to track in the moving direction of the celestial body, and photographing is performed with the celestial body made still by the equatorial telescope. Moreover, in recent years, an imaging apparatus which can take still photos of the heavenly bodies similarly to the equatorial telescope by causing only an image sensor inside the camera to track in the moving direction of the celestial body while the camera is fixed is proposed (see Japanese Patent Laid-Open No. 2012-5112 (hereinafter referred to as “Patent Literature 1”)).
Moreover, an imaging apparatus is also proposed which can generate a composite image equal to the case of taking a still photo of the celestial body by aligning and composing a plurality of images taken in a short exposure time during which movement of the celestial body is not visually recognized (see Japanese Patent Laid-Open No. 2003-259184 (hereinafter referred to as “Patent Literature 2”)).
In recent years, photos of a celestial body with a high image quality can be taken easily with a trend to more pixels and higher image quality of the digital camera, and the number of photographers has increased and the subjects to be taken have a wide variety. Among them, a field called “star-scape photo” in which the celestial bodies (sky/stars) and a view on the earth are contained in one photo has been established.
In order to take this star-scape photo, tracking of the celestial bodies by using the equatorial telescope or the imaging apparatus disclosed in Patent Literatures 1 and 2 can be considered. However, in this case, a still photo of the celestial body which is a major subject can be taken but the view on the earth flows. On the other hand, if a photo is taken by fixing the imaging apparatus, the celestial body flows as described above.
Moreover, in the imaging apparatus in Patent Literature 2, the longer the photographing time becomes, the more the subject flows outside a view angle and thus, the view angle of the composite image becomes narrower. A photographing region which has gone out of the view angle in the middle of photographing can be subjected to exposure correction by gain application to an image output by a rear-stage digital circuit. However, in this case, a noise increases by a portion of an applied gain which results in deterioration of an image quality, and a photo with a view angle or an image quality intended by the photographer cannot be taken.
Moreover, for image composition by shifting the plurality of images so that the position of the celestial body becomes the same, positional information of the celestial body needs to be read out of the image, but the celestial body photographing has an extremely dark subject, and the image subjected to divided exposure is darker and can easily become an image with poorer S/N. It is extremely difficult to accurately read out the positional information of the celestial body from such an image, and it is difficult to generate a composite image of a still celestial body with accuracy.